This invention relates to a printed document of value having at least one authenticity feature in the form of a luminescent substance based on host lattices doped with chromophores with the electron configuration (3d)2.
The term “document of value” refers according to the invention to bank notes, checks, shares, tokens, ID cards, credit cards, passports and other documents as well as labels, seals, packages or other elements for product protection.
Protecting documents of value against forgery by means of luminescent substances has been known for some time. The use of rare earth metals has also been discussed in this context. They have the advantage of having narrow-band characteristic spectral lines that facilitate reliable detection and delimitation over other spectra. The substances preferably used have either absorption or emission outside the visible spectral region.
If the emissions are at wavelengths between about 400 nanometers and about 700 nanometers, the luminescent substances are detectable with the eye upon suitable excitation. This is desirable for some applications, e.g. for an authenticity check by illumination with UV light. For other applications, however, it is of advantage if the emission is outside the visible spectral region since special detectors are then necessary for detecting the substances.
Luminophores with characteristic properties that are suitable for protecting documents of value and in particular for automatic authenticity detection are limited in number, however. Most inorganic and organic luminophores have uncharacteristic, broad spectra and are moreover often commercially available. This impedes their identification and makes it impracticable to use several of said substances simultaneously.